The theory for a light-emitting diode (LED) to emit light is that when a forward voltage power is applied to a p-n junction, the electrons are driven from the n-type semiconductor and the holes are driven from the p-type semiconductor, and these carriers are combined in the active layer to emit light. The efficiency of an LED depends on the Internal Quantum Efficiency (IQE), which depends on the combination rate of the electrons from the n-type semiconductor and the holes from the p-type semiconductor. However, the IQE is reduced by the built-in electric field. The built-in electric field is an effect due to the piezoelectric polarizations as a result of the characteristics of the materials used for LED. For example, an LED which employs nitride films grown along the polar c-direction of a sapphire substrate suffers from the undesirable built-in electric field. The built-in electric field not only reduces the IQE, but also results in the droop effect.
A solution to reduce the built-in electric field in the active layer is to reduce the strain in the active layer. A conventional structure based on this solution is to form a single thick InGaN layer before forming the active layer to reduce the strain.